In general, there has been known a torque distribution control apparatus for a four wheel drive which drives four wheels by engine outputs, wherein the torque distribution for each wheel is not always equivalent but is variably controlled so as to become optimum correspondingly to operating conditions.
For example, Japanese Unexamined Patent Publication No. 1-247223 has disclosed that the turning movement of a car is classified under three parts at the time of turning start, during turning and at the time of turning escape, so that the torque distribution is carried out correspondingly to the turning state of the car. In other words, the torque distribution on the rear wheel side is increased in order to enhance the small turn of the car. In addition, the torque distribution on the front wheel side is increased in order to enhance straightness at the time of turning escape.
Load movement occurs according to the operation of the car such as acceleration, turning and the like. The load movement causes the driving force allowed by the wheels to be ununiform. Japanese Unexamined Patent Publication No. 1-247221 has disclosed that the torque distribution for the four wheels is changed correspondingly to the load movement, the braking force is properly applied to the wheels so as to change the torque distribution, and the engine outputs are increased in order not to cause the entire driving torque to be lowered according to the braking force.
In the case where the torque distribution is changed by combining a plurality of control operations, i.e., torque distribution control corresponding to the turning state of the car and torque distribution control corresponding to the load movement, the torque required to change the torque distribution may not be obtained according to the operating conditions of an engine even if the engine outputs are increased. In this case, when the torque distribution control is executed, the car is decelerated according to the braking force which is applied to the wheels or the torque distribution is made insufficient so that the desired running cannot be obtained.
Depending on a friction coefficient (hereinafter referred to as .mu. if necessary) of a road, there is executed the change of torque distribution which is unnecessary or lowers the running stability of the car. In other words, there is remarkably lowered the responsibility such as the occurrence, decrease or the like of the lateral acceleration or yaw rate for a steering quantity on a low .mu. road. Accordingly, if the distribution corresponding to the load movement is carried out with priority at the time of turning escape in similar to a high .mu. road, the yaw movement is prevented from being decreased. Consequently, the operation of drivers correction is increased. At the time of turning start, it is desired that the lateral torque distribution is carried out so as to quickly cause the lateral acceleration, yaw rate and the like without enhancing grip force of the front wheels by decreasing front wheels driving force.